Watermark embedding method based on DCT subband image characters

ABSTRACT

A watermark embedding method based on DCT subband image characters is disclosed. An original image is converted into a frequency image using a discrete cosine transformation. The watermark information is then embedded into a specific embedding block in each macro-block of the frequency image. The method has the following advantages: (1) the image embedded by watermark is resistant from distortion and damages from compressions; (2) the watermark image can be extracted without employing the original image; and (3) the quality of the image embedded with the watermark image can remain intact.

This Nonprovisional application claims priority under 35 U.S.C. § 119(a)on Patent Application No(s). 092119969 filed in TAIWAN on Jul. 22, 2003,the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to an information embedding method and, inparticular, to a method that embeds a binary watermark image into thesubband image of an original image.

2. Related Art

It is a trend to digitize information nowdays. However, due to the easeof distributing and copying digital images, the protection is thereforenot as easy as convention ones. The copyright of digital data thusreceives much attention. To ensure the security of digital image datatransmissions and to avoid malicious eavesdropping, a widely used methodis the “watermark embedding technique”. The “watermark embeddingtechnique” is to embed in an orignal digital image to be transmittedsome watermark information that is unrelated to the original digitalimage data. Unauthorized third parties are thus unable to take and usethe digital image data. Therefore, currently most digital image data usethis way to prevent eavesdroppers from stealing and damaging theoriginal digital image data.

From the appearance of the watermark printed digital image, one cansimply categorize the watermark information as visible and invisible.From the operational processing, one can divide the watermarkinformation into space domain and frequency domain. However, a goodwatermark embedding technique has to satisfy the following three basicproperties in order to achieve the goal of protecting the originaldigital image data. First, it should be uneasy to detect. That is, theembedded watermark information should not be easy to be discovered bynormal image processing methods. Alternatively, one can try to embed thewatermark information in such a way that only a tiny change is made tothe image and human eyes cannot detect the difference. Secondly, it hasto be robust. The embedded watermark information should be able toresistant to various kinds of attacks, including image processing,magnification, shrinking, rotation, compression and modification.Thirdly, it does not require the original digital image data. Whendetecting or recovering the watermark information, one should not needanother copy of the original digital image data in order to process.

Therefore, it is an important trend and direction for people in thefield of watermark embedding detection technology to develop aneffective and robust watermark embedding technique for all kinds ofdigital image data so that users can rapidly and easily detect thewatermark information and recovering the original digital image.

SUMMARY OF THE INVENTION

In view of the foregoing, the disclosed watermark embedding method fordigital image data is one that is able to perform subband imageprocessing in the frequency domain.

The watermark embedding method based on the DCT subband image characteraccording to the invention contains the following steps. First, a firstexperience parameter TH1 and a second experience parameter TH2 aredetermined. The first parameter TH1 can be set by the required imagedistortion. The second parameter TH2 performs its setting according tothr required image robustness. Afterwards, the frequency image convertedfrom the original image and the binary watermark image series (A1, . . .An) are obtained. The system then extracts the macro-block of thefrequency macro-block and reads the binary watermark image data Anaccording to their order. Afterwards, the system performs comparison theparameter combination and their associated errors in the embedded blockof the macro-block. The parameter combination with the smallest errorsare added and substracted by TH2 (determined by the numerical type) toreplace the discrete cosine parameters in the subband of the embeddedblock. Such parameter setting is performed in each macro-block of thefrequency image until the watermark information is completely embeddedin the image.

With the disclosed method, it is possible to embed and hide thewatermark information in the embedded blocks of specific macro-blocks ofthe frequency image converted from the original image. This cannot onlykeep the original image intact, but is also able to withstand strongdamages. Moreover, the watermark image can be directly extracted withoutthe aid of the original image. Therefore, it is a simple but robust“watermark embedding technique.”

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more fully understood from the detaileddescription given hereinbelow illustration only, and thus are notlimitative of the present invention, and wherein:

FIG. 1 is a flowchart of the disclosed watermark embedding method basedon the DCT subband image characters;

FIG. 2 is a schematic view of embedded block selection according to thedisclosed method; and

FIG. 3 is a schematic view of an embodiment of the invention showing howthe watermark image is embedded.

DETAILED DESCRIPTION OF THE INVENTION

The specification mainly proposes a watermark embedding method based onthe discrete cosine transformation (DCT) characters. The original image900 is first converted into a frequency image 910 using the DCT. Thebinary watermark image (a digital image that can be represented by 0and 1) is then embedded into specific subband image of particularblocks.

Please refer to FIG. 1 for the procedure of the disclosed method. First,a first experience parameter TH1 and a second experience parameter TH2(step 100) are determined. Afterwards, the system obtains the frequencyimage 910 converted from the orignal image 900 and the binary watermarkimage series (A1, . . . An) (step 200). Each of the macro-blocks in thefrequency image 910 and the numerical value of the binary watermarkimage An are extracted. There are many methods available for macro-blockextraction. A preferred embodiment is to extract from left to right(moving to the right by two blocks at a time) and then from top to down(moving down by two blocks at a time). Various parameter combinationsand the least error associated with the DCT in the specific subbandimage of particular embedded blocks are calculated (step 400). If theleast error is smaller than TH1 (step 500) and the numerical value An is1 (or 0), the parameter combination with the least error is added by TH2to replace the DCT parameter of the subband image in the embedded block(step 600). On the other hand, if the numerical value An is 0 (or 1),the parameter combination with the least error is subtracted by TH2 toreplace the DCT parameter of the subband image in the embedded block.

Up to this point, the embedding procedure for a macro-block watermarkimage is completed. The system continues to check whether there is anyother undefined macro-block (step 700). If the answer is yes, then theprocedure goes back to step 300; otherwise, the procedure terminates.

However, if the least error is greater than TH1, it means that thecurrent specific subband image is not suitable for insertinginformation. One then has to perform the step of switching the positionof the specific subband image (step 800), after which the procedurerestarts from step 400. The switching step continues until the mostappropriate specific subband image location is found.

For the computation methods of various parameter combinations, wepropose seven estimation functions as embodiment of the invention.However, these do not exhaust all cases and other functions arepossible. These seven functions are:[Block(1,1)_(D(a,b))+Block(1,2)_(D(a,b))+Block(1,3)_(D(a,b))+Block(2,1)_(D(a,b))+Block(2,3)_(D(a,b))+Block(3,1)_(D(a,b))+Block(3,2)_(D(a,b))+Block(3,3)_(D(a,b))]/8;  (1)[Block(1,1)_(D(a,b))+Block(3,3)_(D(a,b))]/2;   (2)[Block(1,2)_(D(a,b))+Block(3,2)_(D(a,b))]/2;   (3)[Block(2,1)_(D(a,b))+Block(2,3)_(D(a,b))]/2;   (4)[Block(1,3)_(D(a,b))+Block(3,1)_(D(a,b))]/2;   (5)[(Block(1,1)_(D(a,b))+Block(1,2)_(D(a,b))+Block(1,3)_(D(a,b)))/3+(Block(3,1)_(D(a,b))+Block(3,2)_(D(a,b))+Block(3,3)_(D(a,b)))/3]/2;and   (6)[(Block(1,1)_(D(a,b))+Block(2,1)_(D(a,b))+Block(3,1)_(D(a,b)))/3+(Block(1,3)_(D(a,b))+Block(2,3)_(D(a,b))+Block(3,3)_(D(a,b)))/3]/2;  (7)where Block(1,1), Block(1,2), Block(1,3), Block(2,1), Block(2,3),Block(3,1), Block(3,2), and Block(3,3) represent 3*3 macro-blocks from ,and D(a,b) is the DCT parameter of the subband image located at theposition (a,b).

As to the selection of the embedded block among each macro-block, it canbe any one or the central block (the preferred one) in the macro-block.The selection of the specific subband image in each embedded block isusually done by choosing one with the lowest frequency. However, whenthe least error is found to be greater than TH1 in step 600, anothersubband image has to be selected to repeat steps 500 and 600.Reselecting the subband image is performed according to thehigh-frequency order (i.e. scanning in a zigzag way). Theoreticallyspeaking, the frequency of the newly selected subband image is onlylower than the previous subband image. This can avoid the problem ofdifficult extraction of the watermark image as a result of embedding itin a subband image with an extremely high frequency. Of course, thesubband image selection can be performed in a predetermined way. We willuse FIG. 3 to explain in further detail.

Please refer to FIG. 2 for the zigzag scanning scheme. This is awell-known method. Since this type of zigzag scanning can scan in thedirection of increasing frequencies (as shown by the arrow from block 0to block 63), it is very useful for searching the next-to-highestfrequency subband image.

Please refer to FIG. 3 for an embodiment of the invention. We first setthe values of the first experience parameter TH1 and the secondexperience parameter TH2. The original image is then converted by the8*8-block DCT into a frequency image 910.

We set 3*3 blocks as the range for extraction each macro-block. We thenextract each of the 3*3 macro-blocks 920 and read the numerical valuesAn of thye binary watermark image. Afterwards, we begin the embeddingtask for each of the 3*3 macro-blocks 920.

We assign the embedded block 930 as the central block Block(2,2) in each3*3 macro-block 920 and use the DCT parameter of the embedded blockposition (3,2) (i.e. _(D(3,2))) as the embedding position. Afterdetermining the numerical value type that is being read, we followdifferent procedures to determine the steps that should be take to embedthe watermark. When the task of embedding the specific subband images ofthe embedded blocks 930 in all the 3*3 macro-blocks 920 (_(D(3,2)) inthe current embodiment) is done, a frequency image 940 with binarywatermark information is produced. We then use an inverse DCT (IDCT) toconvert the frequency image 940 into an embedded image 950.

In the current embodiment, the method also automatically changes to use^(D(3,3)) as the embedded position when the least error of the DCTparameter of all the parameter combinations and the embedded block 930is greater than the first experience parameter TH1.

Certain variations would be apparent to those skilled in the art, whichvariations are considered within the spirit and scope of the claimedinvention.

1. A watermark embedding method based on discrete cosine transformation (DCT) subband image characters to generate a frequency image from an original image using an 8*8-block DCT and to embed a binary watermark image into a subband image of the frequency image, the method comprising the steps of: setting the values of a first experience parameter TH1 and a second experience parameter TH2; obtaining the frequency image converted from the original image and a series (A1 . . . An) of the binary watermark image; extracting in order a 3*3 macro-block of the frequency image and read in order the numerical value An of the binary watermark image; and analyzing the type of the numerical value being read from the binary watermark image and performing a DCT parameter setting for an embedded block, which further includes the steps of: computing a plurality of sets of parameter combinations of the embedded block in the 3*3 macro-block and comparing the parameter combinations with the DCT parameter error of the embedded block; adding the parameter combination with the least error and TH2 to replace a DCT parameter of the subband image position in the embedded block when the least error is less than TH1 and An is one of 1 or 0; and subtracting TH2 from the parameter combination with the least error to replace a DCT parameter of the subband image position in the embedded block when the least error is less than TH1 and An is the other of 1 or
 0. 2. The method of claim 1, wherein the first experience parameter TH1 is set according to required image distortion.
 3. The method of claim 1, wherein the second experience parameter TH2 is set according to required image robustness.
 4. The method of claim 1, wherein the extraction of the 3*3 macro-block is performed from left to right (two blocks to the right at a time) and from top to bottom (two blocks down at a time).
 5. The method of claim 1, wherein the embedded block is the central block Block(2,2) of the 3*3 macro-block.
 6. The method of claim 1, wherein the estimation function of the parameter combinations is selected from the group consisting of: [Block(1,1)_(D(a,b))+Block(1,2)_(D(a,b))+Block(1,3)_(D(a,b))+Block(2,1)_(D(a,b))+Block(2,3)_(D(a,b))+Block(3,1)_(D(a,b))+Block(3,2)_(D(a,b))+Block(3,3)_(D(a,b))]/8;  (1) [Block(1,1)_(D(a,b))+Block(3,3)_(D(a,b))]/2;  (2) [Block(1,2)_(D(a,b))+Block(3,2)_(D(a,b))]/2;  (3) [Block(2,1)_(D(a,b))+Block(2,3)_(D(a,b))]/2;  (4) [Block(1,3)_(D(a,b))+Block(3,1)_(D(a,b))]/2;  (5) [(Block(1,1)_(D(a,b))+Block(1,2)_(D(a,b))+Block(1,3)_(D(a,b)))/3+(Block(3,1)_(D(a,b))+Block(3,2)_(D(a,b))+Block(3,3)_(D(a,b)))/3]/2; and  (6) [(Block(1,1)_(D(a,b))+Block(2,1)_(D(a,b))+Block(3,1)_(D(a,b)))/3+(Block(1,3)_(D(a,b))+Block(2,3)_(D(a,b))+Block(3,3)_(D(a,b)))/3]/2;  (7) where Block(1,1), Block(1,2), Block(1,3), Block(2,1), Block(2,3), Block(3,1), Block(3,2), and Block(3,3) represent the 3*3 macro-blocks from left-top to right-bottom, and _(D(a,b)) is the DCT parameter of the subband image located at the position (a,b).
 7. The method of claim 6, wherein a=3 and b=2.
 8. The method of claim 6, wherein the subband image position is changed to a=3 and b=3 if the least error of a=3 and b=2 is greater than the first experience parameter TH1.
 9. The method of claim 1, wherein adjacent 3*3 macro-blocks allow overlap 1*3 blocks on top/bottom or 3*1 blocks on left/right.
 10. A watermark embedding method based on DCT subband image characters to generate a frequency image from an original image using a p*p block DCT and to embed a binary watermark image in a subband image of the frequency image, the method comprising the steps of: setting the values of a first experience parameter TH1 and a second experience parameter TH2; obtaining the frequency image converted from the original image and a series (A1 . . . An) of the binary watermark image; extracting in order a q*q macro-block of the frequency image and read in order the numerical value An of the binary watermark image; and analyzing the type of the numerical value being read and performing a DCT parameter setting for an embedded block, which further includes the steps of: computing a plurality of sets of parameter combinations of the embedded block in the q*q macro-block and comparing the parameter combinations with the DCT parameter error of the embedded block; adding the parameter combination with the least error and TH2 to replace a DCT parameter of the subband image position in the embedded block when the least error is less than TH1 and An is one of 1 or 0; and subtracting TH2 from the parameter combination with the least error to replace a DCT parameter of the subband image position in the embedded block when the least error is less than TH1 and An is the other of 1 or
 0. 11. The method of claim 10, wherein the first experience parameter TH1 is set according to required image distortion.
 12. The method of claim 10, wherein the second experience parameter TH2 is set according to required image robustness.
 13. The method of claim 10, wherein the extraction of the q*q macro-block is performed from left to right (two blocks to the right at a time) and from top to bottom (two blocks down at a time).
 14. The method of claim 10, wherein the embedded block is an arbitrary block Block(i,j) of the q*q macro-block.
 15. The method of claim 10, wherein adjacent q*q macro-blocks allow overlap 1*q blocks on top/bottom or q*1 blocks on left/right. 